1. Field of the Invention
The present invention relates to a single-mode optical fiber usable as a long-haul transmission line for optical communications and the like; and, in particular, to a dispersion-shifted fiber suitable for large-capacity optical communications-such as wavelength division multiplexing (WDM) transmission and the like.
2. Related Background Art
Conventionally, light in a 1.3-xcexcm wavelength band or 1.55-xcexcm wavelength band has often been utilized as light signals for communications in optical communications systems employing single-mode optical fibers as their transmission lines. Recently, however, the use of light in 1.55-xcexcm wavelength band has been increasing from the viewpoint of lowering transmission loss in transmission lines. Single-mode optical fibers employed in such a transmission line for light in the 1.55-xcexcm wavelength band (hereinafter referred to as 1.55-xcexcm single-mode optical fibers) have been designed such that their chromatic dispersion (phenomenon in which pulse waves widen because of the fact that the propagation speed of light varies depending on wavelength) with respect to light in the 1.55-xcexcm wavelength band becomes zero (so as to yield dispersion-shifted fibers having a zero-dispersion wavelength of 1.55 xcexcm).
As such a dispersion-shifted fiber, Japanese Patent Application Laid-Open No. HEI 8-304655 (U.S. Pat. No. 5,613,027) and U.S. Pat. No. 5,659,649, for example, propose a dispersion-shifted fiber having a refractive index profile of a ring-like core structure, whose core region is constituted by an inner core and an outer core having a higher refractive index than the inner core. Also, Japanese Patent Application Laid-Open No. HEI 8-248251 (European Patent Publication No. 0 724 171 A2) and Japanese Patent Application Laid-Open No. HEI 9-33744 propose a dispersion-shifted fiber having a refractive index profile of a dual ring core structure, whose core region is constituted by a first core, a second core having a higher refractive index than the first core, a third core having a lower refractive index than the second core, and a fourth core having a higher refractive index than the third core.
On the other hand, Japanese Patent Application Laid-Open- No. SHO 63-43107 and Japanese Patent Application Laid-Open No. HEI 2-141704 propose a depressed cladding structure whose cladding region is constituted by an inner cladding and an outer cladding having a higher refractive index than the inner cladding.
In recent years, the advent of wavelength division multiplexing (WDM) transmission and optical amplifiers has further enabled long-haul transmission, and various improvements have been made as to optical fibers such as those mentioned above in order to avoid nonlinear phenomena. Here, nonlinear optical effects refer to phenomena in which light signal pulses distort in proportion to the density of light intensity or the like due to nonlinear phenomena such as four-wave mixing (FWM), self-phase modulation (SPM), cross-phase modulation (XPM), and the like, and become a factor restricting the transmission speed and the repeater spacing in repeating transmission systems.
In general, the amount of occurrence of nonlinear phenomena has been known to be proportional to the amount of change in refractive index given by the following expression (1):
(N2/Aeff)xc3x97Pxe2x80x83xe2x80x83(1)
where N2 is the nonlinear refractive index (unit: m2/W) , Aeff is the effective area (unit: xcexcm2), and P is the optical power.
Here, the nonlinear refractive index N2 is defined as follows. Namely, the refractive index  less than N greater than  of a medium under strong light varies depending on the optical power. Therefore, the lowest-order effect on this refractive index  less than N greater than  is given by the following expression (2):
xe2x80x83 less than N greater than = less than N0 greater than + less than N2 greater than xc2x7Ixe2x80x83xe2x80x83(2)
where  less than N0 greater than  is the refractive index with respect to linear polarization,  less than N2 greater than  is the nonlinear refractive index with respect to nonlinear polarization, and I is the light intensity. Under strong light, the refractive index  less than N greater than  of the medium is given by the sum of its normal value  less than N0 greater than  and the increase proportional to the light intensity. In particular, the constant of proportion  less than N2 greater than  (unit: m2/W) in the second term is known as nonlinear refractive index.
On the other hand, as shown in Japanese Patent Application Laid-Open No. HEI 8-248251 (EP 0 724 171 A2), the effective area Aeff is given by the following expression (3):                               A          eff                =                  2          ⁢                      xe2x80x83                    ⁢                                                    π                ⁡                                  (                                                            ∫                      0                      ∞                                        ⁢                                                                  E                        2                                            ⁢                      r                      ⁢                                              xe2x80x83                                            ⁢                                              ⅆ                        r                                                                              )                                            2                        /                          (                                                ∫                  0                  ∞                                ⁢                                                      E                    4                                    ⁢                  r                  ⁢                                      xe2x80x83                                    ⁢                                      ⅆ                    r                                                              )                                                          (        3        )            
where E is the electric field accompanying the propagating light, and r is the radial distance from the core center (the center axis of the optical fiber).
Each of the above-mentioned conventional optical fibers has been designed so as to enhance the effective area Aeff in order to suppress the occurrence of nonlinear phenomena. However, the inventors have studied the conventional optical fibers and, as a result, have found problems as follows. Namely, there is inevitably a limit to the enhancement of effective area Aeff since it increases the transmission loss upon bending the optical fiber at a predetermined radius (hereinafter referred to as macrobending loss) and the transmission loss due to an external pressure (side pressure) applied to the side face of the optical fiber (hereinafter referred to as microbending loss). Though the microbending loss of optical fibers having a refractive index profile of a ring-like core structure is lower than that of optical fibers having other refractive index profiles such as those of dual ring core and multilayer core structures in general, there is still an upper limit to the above-mentioned enhancement. Thus, the increase in macrobending loss and microbending loss along with the enhancement of effective area Aeff is an essential problem of optical fibers, which is inescapable.
In order to overcome problems such as those mentioned above, it is an object of the present invention to provide an optical fiber comprising a structure which effectively suppresses the occurrence of nonlinear phenomena without increasing transmission loss such as macrobending loss.
The optical fiber according to the present invention is suitable for a single-mode optical fiber which is mainly composed of silica glass and which has a core region extending along a predetermined axis and a cladding region provided on the outer periphery of the core region. Also, in view of its application to the wavelength division multiplexing transmission and the like in recent years, the optical fiber according to the present invention has, with respect to light having a wavelength of 1550 nm, a dispersion with an absolute value of 1.0 to 4.5 ps/nm/km and an effective area of 70 xcexcm2 or more, and has a cutoff wavelength of 1.3 xcexcm (1300 nm) or more at a fiber length of 2 m.
Specifically, as shown in FIGS. 1A and 1B, the optical fiber according to the present invention comprises, at least, a center region 101 extending along a center axis and having a predetermined refractive index; a first annular region 102 provided on the outer periphery of the center region 101 and having a higher refractive index than the center region 101; and a second annular region 103 provided on the outer periphery of the first annular region 102 and having a lower refractive index than the first annular region 102.
In particular, in order to lower the nonlinear refractive index N2 so as to suppress the occurrence of nonlinear phenomena (see the above-mentioned expression (1)), contents of fluorine, which is a refractive index reducing dopant, in the second annular region 103 is adjusted such that the refractive index thereof radially decreases from the center of the optical fiber 100. Also, in the optical fiber according to the present invention, the radius of the first annular region 102 is set to 1.5 xcexcm or more in particular in order to keep the optical power from concentrating near the center axis. Here, the relationship between the effective area Aeff given by the above-mentioned expression (3) and the mode field diameter MFD is given by the following expression (4):
Aeff=kxc2x7(xcfx80/4)xc2x7(MFD)2xe2x80x83xe2x80x83(4)
and the constant of proportion k in this expression (4) is preferably 1.4 or more.
As shown in T. Kato, et al., xe2x80x9cEstimation of nonlinear refractive index in various silica-based glasses for optical fibersxe2x80x9d (OPTICS LETTERS, Vol. 20, No. 22, Nov. 15, 1995), the dependence of nonlinear refractive index N2 upon fiber compositions has been known to be the lowest in pure silica and increase as impurities such as fluorine are added thereto. On the other hand, among the individual regions of the optical fiber, the contribution of the first annular region 102 to the nonlinear refractive index N2 is greater than that of the second annular region 103, and the first annular region 102 also greatly contributes to determining the above-mentioned various characteristics of the optical fiber at a wavelength of 1550 nm.
Therefore, the present invention takes account of the second annular region 103 positioned outside the first annular region 102. Specifically, the contents of fluorine is controlled such that the shape of the refractive index profile in the second annular region 103 is tilted from the center of the optical fiber 100 to ward its perimeter. Namely, while being made smaller in an inner part of the second annular region 103 in which the light propagating through the optical fiber 100 has a higher power, the fluorine contents are increased toward an outer part of the second annular region 103 (as the optical power of propagating light weakens), so as to intentionally tilt the refractive index profile in the second annular region 103, thereby lowering the nonlinear refractive index N2 of the whole optical fiber.
From results of experiments which will be mentioned later, the inventors have found it preferable to tilt the refractive index profile in the second annular region 103 such that the relative refractive index difference of the second annular region 103 with respect to a reference glass region lowers by 0.02% or more per distance of 1 xcexcm from the center of the optical fiber. In the case where the shape of refractive index profile in the second annular region 103 is not linear, the value (%/xcexcm) indicating the gradient of refractive index profile is given by the gradient of an approximate line obtained by the method of least squares or the like.
Various modes are applicable to the structure of refractive index profile realizing the optical fiber according to the present invention. Namely, ring-like core structures, dual ring structures, multilayer core structures, and the like are employable as the structure of refractive index profile in the core region of the optical fiber, whereas depressed cladding structures are employable as the refractive index profile in the cladding region.
For example, in the case where a refractive index profile of a ring-like core/depressed cladding structure such as the one 250 shown in FIGS. 2A and 2B is employed as the refractive index profile of the optical fiber according to the present invention, the center region 101 corresponds to the inner core 211 extending along the center axis of the optical fiber 200, the first annular region 102 corresponds to the outer core 212 provided on the outer periphery of the inner core 211, and the second annular region 103 corresponds to the inner cladding 221 provided on the outer periphery of the outer core 212. Here, a refractive index difference is given to each of the glass regions 211, 212, 221 with reference to the outer cladding 222 provided on the outer periphery of the inner cladding 221.
In the case where a refractive index profile of a dual ring core structure such as the one 450 shown in FIGS. 7A and 7B is employed as the refractive index profile of the optical fiber according to the present invention, the center region 101 corresponds to the first core 411 extending along the center axis of the optical fiber 400, the first annular region 102 corresponds to the second core 412 provided on the outer periphery of the first core 411, and the second annular region 103 corresponds to the third core 413 provided on the outer periphery of the second core 412. In the optical fiber 400 of FIGS. 7A and 7B, the fourth core 414 is provided on the outer periphery of the third core 413, and the cladding region 420 is provided on the outer periphery of the fourth core. A relative refractive index difference is given to each-of the glass regions 411, 412, 413, 414 with reference to the cladding region 420. This refractive index profile 450 may also employ a depressed cladding structure.
The present invention will be more fully understood from the detailed description given herein below and the accompanying drawings, which are given by way of illustration only and are not to be considered as limiting the present invention.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will be apparent to those skilled in the art from this detailed description.